Swagger API documentation in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Swagger setup in NestJS?

Given this NestJS Swagger setup, what will the generated API documentation show for the GET /cats endpoint?

NestJS

import { Controller, Get } from '@nestjs/common';
import { ApiTags, ApiOperation } from '@nestjs/swagger';

@ApiTags('cats')
@Controller('cats')
export class CatsController {
  @Get()
  @ApiOperation({ summary: 'List all cats' })
  findAll() {
    return ['Tom', 'Garfield'];
  }
}

A The Swagger UI will show a 'cats' section with a GET /cats endpoint labeled 'List all cats'.

B The Swagger UI will show no endpoints because @ApiTags is missing.

C The Swagger UI will show a POST /cats endpoint instead of GET.

D The Swagger UI will show the GET /cats endpoint but with no description.

💡 Hint

Look at the decorators @ApiTags and @ApiOperation and what they do.

The @ApiTags('cats') decorator groups the controller endpoints under the 'cats' tag in Swagger UI. The @ApiOperation decorator adds a summary description to the GET /cats endpoint. So the Swagger UI will show a 'cats' section with the GET /cats endpoint labeled 'List all cats'.

📝 Syntax

intermediate

Which option correctly adds a query parameter to Swagger docs in NestJS?

You want to document a query parameter 'limit' of type number for a GET endpoint using Swagger decorators in NestJS. Which option correctly adds this parameter?

NestJS

import { Controller, Get, Query } from '@nestjs/common';
import { ApiQuery } from '@nestjs/swagger';

@Controller('items')
export class ItemsController {
  @Get()
  // Add query param decorator here
  findAll(@Query('limit') limit: number) {
    return [];
  }
}

A @ApiQuery('limit')

B @ApiQuery({ name: 'limit', type: 'string', required: true })

C @ApiQuery({ name: 'limit', type: Number, required: false })

D @ApiParam({ name: 'limit', type: Number })

💡 Hint

Check the correct decorator for query parameters and the type property.

The @ApiQuery decorator documents query parameters. The type should be Number (the class, not string). Option C correctly uses @ApiQuery({ name: 'limit', type: Number, required: false }). Option C uses wrong type and required true. Option C is invalid syntax. Option C uses @ApiParam which is for path parameters, not query.

🔧 Debug

advanced

Why does this Swagger setup not show the expected response schema?

This NestJS controller method uses @ApiResponse but Swagger UI does not show the response schema. What is the cause?

NestJS

import { Controller, Get } from '@nestjs/common';
import { ApiResponse, ApiProperty } from '@nestjs/swagger';

class Cat {
  @ApiProperty()
  name: string;

  @ApiProperty()
  age: number;
}

@Controller('cats')
export class CatsController {
  @Get()
  @ApiResponse({ status: 200, description: 'Success', type: Cat })
  findAll() {
    return [{ name: 'Tom', age: 5 }];
  }
}

A The Cat class lacks decorators like @ApiProperty, so Swagger cannot generate schema.

B The return type must be an array of Cat, but type: Cat is used.

C The controller method must be async to show response schema.

D The @ApiResponse decorator is used incorrectly; it should be @ApiOkResponse.

💡 Hint

Swagger needs metadata on class properties to generate schemas.

Swagger uses decorators like @ApiProperty on class properties to generate response schemas. Without these, the schema is missing. Option A is incorrect because @ApiResponse can be used with type. Option A is irrelevant. Option A is not mandatory; Swagger can infer arrays but needs property metadata.

🧠 Conceptual

advanced

What is the purpose of the SwaggerModule in NestJS?

In NestJS, what does the SwaggerModule do when setting up API documentation?

A It automatically generates API controllers without writing code.

B It creates and serves the Swagger UI and JSON docs based on decorators in the code.

C It replaces the NestJS routing system with Swagger routes.

D It validates incoming API requests against Swagger specs at runtime.

💡 Hint

Think about how SwaggerModule interacts with your NestJS app and decorators.

The SwaggerModule reads the decorators in your NestJS code and generates the Swagger JSON and UI to document your API. It does not generate controllers, replace routing, or validate requests at runtime.

❓ state_output

expert

What is the output of this NestJS Swagger setup with multiple tags and global prefix?

Consider this NestJS app setup with Swagger and a global prefix. What URL will serve the Swagger UI?

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { SwaggerModule, DocumentBuilder } from '@nestjs/swagger';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.setGlobalPrefix('api/v1');

  const config = new DocumentBuilder()
    .setTitle('My API')
    .setDescription('API description')
    .setVersion('1.0')
    .addTag('users')
    .addTag('products')
    .build();

  const document = SwaggerModule.createDocument(app, config);
  SwaggerModule.setup('docs', app, document);

  await app.listen(3000);
}
bootstrap();

A Swagger UI will not be available because of the global prefix

B Swagger UI will be available at http://localhost:3000/api/v1/docs

C Swagger UI will be available at http://localhost:3000/api/v1/swagger

D Swagger UI will be available at http://localhost:3000/docs

💡 Hint

Check how SwaggerModule.setup path relates to global prefix.

The global prefix applies to the app routes, but SwaggerModule.setup serves the UI at the exact path given relative to the root. So Swagger UI is at /docs, not under the global prefix /api/v1.